With these times I can jump through ad's pretty well...
Sometimes though I might jump forward (say) 30 sec's and find I've gone too far, so I'd jump back 20sec, and then if that's too far back I'd jump forward 10 sec's.... an aggregate of 10 Sec's back... Simple. However...

When I did the back 30, forward 10, I'd find myself back pretty much where I was before I did the backward jump. So... I made myself a little video of a time display incrementing every 100mS, in the form where [ 0:15:08.5 ] would show 15Min's 8½ Sec's. Now I can see what is happening...

I find in order to achieve my required timing (close enough) I need to set the jumps thus: -

Jump times are really approximate. As I understand it, one reason is that the transmitted signal only contains a complete picture at intervals. After a complete picture only the *differences* are transmitted. So any jump has to find the nearest (? or previous, or next?) complete picture before it can start the display. The intervals between complete pictures vary with program and channel, as it affects the overall data rate.

I'm fairly certain that it always jumps to the *next* GOP, which is why forward jumps turn out to be bigger than backward jumps of the "same size". Note that GOPs vary in size depending on channel, so jumps that work a particular way on one channel may not work the same way on another.

I'm fairly certain that it always jumps to the *next* GOP, which is why forward jumps turn out to be bigger than backward jumps of the "same size". Note that GOPs vary in size depending on channel, so jumps that work a particular way on one channel may not work the same way on another.

Julie

Thanks Julie, What's a GOP ?
Do you mean TV channel? e.g. (say) BBC1 and Channel 4 ?
Yes, I find that on some channels, 'Back 20' doesn't seem to go back as far as I expect, even after my 'time tweak'.

Yes, I do mean TV channels. BBC channels typically have higher bit rates, so the GOP boundaries are closer together, resulting in more accurate jumps than on channels with lower bit rates where the GOP boundaries are further apart.